This invention generally relates to manually operable gear shift mechanisms for automotive multi-speed transmissions and particularly to an improved arrangement for eliminating failure in manual gear shifting between one forward speed and a reverse speed.
Gear shifting in this type of transmission is usually accomplished with a floor-type gear shift lever. The shift lever is connected with a well known striking rod which is axially reciprocal and turnable in accordance with movement of the shift lever in two crossing directions. Integrally mounted on the striking rod is a shorter selector rod depending therefrom, below which a plurality of spaced, parallel fork shafts is arranged. The selector rod is pivotal with turning movement of the striking rod for selective engagement of the end piece thereof in one of recesses machined to the respective inner ends of the fork shafts. The selection of the recess depends on various angles through which the striking rod is turned. With a reciprocal movement of the striking rod, the fork shaft engaged with the selector rod is moved usually in opposite directions providing two different speeds.
Taking an example of a transmission with five forward and one reverse speeds, the end piece of the selector rod fits in one of three recesses respectively of three fork shafts. When the end piece is in the first recess, the corresponding fork shaft is axially moved in the opposite direction to selectively provide the first and second forward speeds. Similarly, the selector rod in the second recess allows the corresponding fork shaft to move between the third and fourth speeds. With the end piece received in the third recess, the movement of the third fork shaft in one direction provides the fifth forward speed, while the movement made by the same fork shaft in the opposite direction provides the reverse speed.
It sometimes occurs in this type of gear shift mechanisms that upon shifting from the fifth to the lower forward speed, the fork shaft will run beyond the neutral position toward the reverse position due to overshifting or other causes. This will result in grating of the gears and often in mechanical damages to the gearing.
In order to eliminate the aforementioned problem, one prior art example proposes to mount another separate fork shaft exclusively for the reverse speed. This shaft has its own recess in which the end piece of the selector rod is engaged. Thus, the reverse speed is reached only when the selector rod is further swung by a predetermined angle out of the third recess to be completely received in the fourth recess so that failure due to careless shift operation is nearly impossible. This however entails a heavier and more troublesome shift operation.
There is another known example which provides a stop mechanism limiting the movement of the fork shaft beyond the neutral position toward the reverse speed. In this example, the gear shift mechanism should further install a particularly designed position determining arrangement for providing two different positions of the selector rod in dependence on whether the fifth speed or the reverse speed has been selected: In the first shift position of the selector rod determined by the position determining mechanism, the fork shaft is moved to the fifth speed while being prevented from moving to the reverse speed by means of the stop mechanism. In the second position selected, the fork shaft is permitted to move to the reverse speed avoiding the stop mechanism. Although this prior art mechanism can achieve an easier gear shifting it is impractical that a complicated arrangement of position determining mechanism is additionally required.